1. Field of the Invention
The present invention generally relates to a method for producing a silver conductive film. More specifically, the invention relates to a method for producing a conductive film of silver used for forming conductive circuits of electronic parts and so forth, such as an RFID antenna.
2. Description of the Prior Art
Conventionally, electric wirings and conductive circuits of electronic parts, such as RFID antennas, which are required to have high reliability, are formed on masked substrates by sputtering expensive noble metals. However, in the method for forming the electric wirings and conductive circuits by sputtering, it is required to carry out various processes, so that its productivity is not high. In addition, a part of the expensive noble metal fed as a raw material is not used for forming the electric wirings and conductive circuits, so that it is studied to form the electric wirings and conductive circuits by other methods from the point of view of the effective practical use of natural resources.
In recent years, as methods for easily mass-producing electric wirings and conductive circuits of electronic parts, there are noted printed electronics for applying a printing technique to form electric wirings and conductive circuits, and it is studied that electric wirings and conductive circuits are formed by sintering metal particles with each other after a conductive ink, which contains the metal particles dispersed in a dispersing medium, is printed on a substrate by printing technique, such as flexographic printing or screen printing.
If the particle diameter of metal particles is in the range of about a few nanometers to about tens nanometers, the specific surface area thereof is very large, so that the melting point thereof is dramatically low. For that reason, a conductive ink, which contains such fine metal particles dispersed in a dispersing medium, can be used for forming finer electric wirings and conductive circuits than a conductive ink which contains metal particles having a particle diameter of about a few micrometers dispersed in a dispersing medium. In addition, the fine metal particles can be sintered with each other at a low temperature of not higher than 200° C., so that it is possible to use various substrates, such as substrates having low heat resistance. For that reason, it is expected that a conductive ink (a fine metal particle dispersing solution), which contains fine metal particles (metal nanoparticles) having a particle diameter of not greater than tens nanometers dispersed in a dispersing medium, is applied to printed electronics for forming fine electric wirings and conductive circuits of electronic parts.
Fine metal particles having a particle diameter of not greater than tens nanometers have very high activity, so that they are unstable particles as they are. For that reason, in order to prevent fine metal particles from being sintered and aggregated with each other for ensuring the independence and storage stability of fine metal particles, there is proposed a conductive ink (a fine metal particle dispersing solution) containing fine metal particles which are coated with an organic substance, such as a long-chain surface active agent, and which are dispersed in an organic solvent, such as decane or terpineol. However, if fine metal particles are coated with a high-molecular weight long-chain surface active agent, it is required to carry out a heat treatment at a high temperature in order to remove and decompose the surface active agent on the surface of the fine metal particles when the fine metal particles are sintered with each other to form a metal conductive film to form electric wirings and conductive circuits, since the boiling point and decomposition point of the surface active agent are high. In addition, it is not possible to use substrates having low heat resistance, and it is required to carry out a heat treatment for a relatively long period of about 30 minutes to 1 hour, so that its productivity is bad. Moreover, if an organic solvent is used as the dispersing medium of the conductive ink, environmental pollution may be caused unless the organic solvent is carefully disposed, and vaporized organic components are diffused to environment when the organic solvent is heated and/or allowed to stand in an open system. For that reason, it is required to install a local ventilation system and so forth when a large amount of organic solvent is disposed, so that it is desired to use a dispersing medium, the main component of which is not any organic solvent, from the point of view of environment and operation.
Thus, there is proposed a metal nanoparticle composition wherein metal nanoparticles coated with any one of straight-chain fatty acids having 3-8 carbon atoms or a derivative thereof are dispersed in a medium containing water as a main body (see, e.g., Japanese Patent Laid-Open No. 2011-202265). This metal nanoparticle composition can be used for sintering metal nanoparticles with each other to form good electric wirings, conductive circuits and so forth on a substrate by carrying out a heat treatment at a low temperature of, e.g., not higher than 140° C., for a short period of, e.g., shorter than 90 seconds.
There are also proposed a method for causing ultrafine metal particles, which are dispersed as metal colloid in water solvent or an organic solvent, to act upon a compound, which has a halogen in its molecule with ionic bond, to obtain electrical conductivity on a substrate (see, e.g., Japanese Patent Laid-Open No. 2008-4375), and a method for applying an ultrafine silver particle containing composition, which contains ultrafine silver particles having a particle size of not greater than 0.1 micrometer, a polymeric latex and a water-soluble halide in a water based medium, to the surface of a substrate to dry the composition to prepare a conductive pattern (see, e.g., Japanese Patent Laid-Open No. 2011-159392). In these methods, it is possible to form electric wirings, conductive circuits and so forth on substrates without the need of any burning steps.
However, if the metal nanoparticle composition disclosed in Japanese Patent Laid-Open No. 2011-202265 is printed on a substrate, such as a PET film or paper, which is inexpensive and which has low heat resistance, by a continuous roll-to-roll flexographic printing machine, which is industrially used in general, to be heat-treated, it is not possible to sufficiently sinter metal nanoparticles with each other, so that it is difficult to obtain good electrical conductivity. That is, if the printing speed is increased to enhance productivity in the continuous roll-to-roll flexographic printing machine, even if the temperature in a heat treat furnace attached to the flexographic printing machine is set at 140° C. (in order to sinter the metal nanoparticles of the metal nanoparticle composition, which is disclosed in Japanese Patent Laid-Open No. 2011-202265, with each other), the substrate is delivered from the heat treat furnace before it is heated to that temperature. On the other hand, if the set temperature is raised, there is a problem in that the substrate is deformed or burned by heat even if the metal nanoparticles can be burned with each other. For that reason, it is not possible to sufficiently sinter the metal nanoparticles with each other, so that it is difficult to obtain good electrical conductivity.
In the methods disclosed in Japanese Patent Laid-Open Nos. 2008-4375 and 2011-159392, the composition or the like applied on the substrate is dried to form the conductive pattern thereon without the need of any burning steps, so that it is difficult to obtain good electrical conductivity by printing on the substrate by means of a continuous roll-to-roll flexographic printing machine.